Water-flooding extraction is one of the main oil field exploitation methods. However, long-term water-flooding extraction in oil fields results in aggravated non-homogeneity of the strata. accelerated water cut rising and degraded water flooding efficiency or inefficient circulation in the middle and late stages of oil field exploitation. Consequently, a great deal of oil remaining in the strata can't be recovered. Hence, it is a prerequisite to modify and control the non-homogeneity of the strata, in order to improve the result of water-flooding extraction of oil reservoirs in the middle and late stages.
Among the various technical measures, injecting polymers, gel plugging agents, polymeric microspheres, and pre-cross-linked particles, etc. are important technical means to realize modification and control of oil reservoirs. However, some problems incurred by the above-mentioned technical measures have been exposed in the implementation process at oil reservoir mining sites. Affected by factors such as shearing action of ground surface injection equipment, shearing action of formation infiltration flow, physical and chemical properties of formation (temperature, salinity, pH, etc.), and dilution by formation water, etc., the viscosity loss of the polymer is severe and the mobility control capability is weakened in the polymer flooding process, especially, in the follow-up water flooding stage, the injection pressure drops rapidly, and it is difficult to achieve a long-time effect modification and control effect; affected by the polymers, the gelling fluid has uncertain gelation time and compromised gelation strength when it flows in porous media, consequently, the modification and control effect is degraded, the shearing effect is more obvious for low-permeability oil reservoirs; in addition, polymeric microspheres are prepared from monomer (AM) raw materials, have demanding requirements for the environment, besides, the required injected amount of polymeric microspheres is large, the cost is high, and the preparation process is relatively complex; the size distribution of pre-cross-linked particles is mainly at millimeter level; the pre-cross-linked particles enter into the formation mainly in a broken form under the formation pressure when they migrate to the deep portion of the formation, pre-cross-link particles are especially unsuitable for deep modification and control of low-permeability oil reservoirs.
CN102936490A disclosed a method for preparing an environment-friendly multi-scale zirconium dispersed particle gel plugging agent, which employ some cross-linking techniques and dispersion techniques. Specifically, specific bulk gel is formed on the ground surface and is sheared mechanically and rounded physically, and then a homogeneously dispersed aqueous phase solution with different size distributions is obtained. The zirconium dispersed particle gel prepared with that method is insusceptible to the uncontrollable reservoir conditions during the gelation reaction, can meet the requirement of large-scale industrial production, and is environment-friendly. Nanometer-sized, micrometer-sized, and millimeter-sized zirconium dispersed particle gels obtained with that method can enter into the deep portion of the formation by virtue of their particle size, and accumulate and swell in the deep portion of the formation, thereby can effectively modify the water injection profile of the formation and have strong mobility control capability. The bulk gel used for preparing the dispersed particle gel with the method is formed by polymers and a zirconium cross-linking agent. The system can be turned into gel rapidly at room temperature, but the zirconium bulk gel formed in that way has poor viscoelasticity; in addition, most of the multi-scale dispersed particle gels prepared in that way are only applicable to oil reservoirs that have medium or low temperature (≤60° C.) and medium or low salinity (530,000 mg/L).
CN106047324A disclosed a deep profile control and flooding agent for strengthening dispersed particle gel, which is applicable to oil reservoirs with low permeability, high temperature, and high salinity. The bulk gel system involved in the method disclosed in the patent document is formed by functional polymers, an aldehyde cross-linking agent, a phenol cross-linking agent, and nano-silica, and the multi-scale dispersed particle gel prepared with the method is applicable to high-temperature and high-salinity oil reservoirs. However, the bulk gel prepared with that method requires high gelation temperature (130° C.) and long gelation time (≥12 h), i.e., the gelation conditions are relatively demanding. The excessively high temperature is adverse to the field operation, and the excessively long gelation time leads to excessive energy consumption. Affected by the demanding gelation conditions and long gelation time, the rigid nano-silica particle system added into the gelling fluid may coagulate and precipitate easily, and consequently the difficulty in mechanical shearing is increased. In addition, owing to a fact that the shearing clearance between the stator and the rotor of the mechanical shearing apparatus (e.g., colloid mill) is very small, the rigid nano-silica particle system may cause damages to the apparatus in the high-speed shearing process of the bulk gel; as a result, the service life of the mechanical shearing apparatus may be shortened.
CN1069116249A disclosed a gel plugging agent applicable to water plugging and injection profile modification for low-temperature high-salinity oil reservoirs. The gel plugging agent comprises acrylamide, N,N-methylene-bis acrylamide, ammonium persulfate, sodium ferricyanide, and water that accounts for the remaining content, but the gelation time is 18-62 h, and the entire preparation process of the dispersed particle gel takes 1-6 h. The gelation time of the gel prepared with that method is too long to meet the requirement for large-size field preparation of dispersed particle gel.